External earth fault loop tt system

[Deleted member 9648]

I think the 1667 is stupid, it's like someone saying phew at least the RCD will work quick lets go..........It's like going back to previous versions of the regs 15th if memory serves me correctly and using the "alternative method" 50v rule ............personally I would feel much happier if everyone aimed at as low ra as they can, much lower than 200. I for one would be aiming at well below 10 ohms at the very least. If the DNO could only give us 10 ohms on TN supplies there would be an outcry, we'd all have to fit RCD's at every installation's origin. I've worked on many large sites over the years and the engineer on site would never except a ra of 200 let alone 1667 they would always be aiming way lower than 100. 5 - 10 would be accepted after some hard graft but as has been said it's possible to achieve below an ohm in a lot of cases with a bit of hard work with some decent rods. Lower the better IMO

I dont disagree up to a point and I think you will find Skelton doesnt either.
The issue constantly raised by us is that many who scoff at the Bs7671 max values appear to be happy with values which will still not operate an OCPD in a reasonable time, if at all.They are still reliant on an RCD.
Given that a value of 10 ohms or 200 ohms......or even 1667 ohms will make no difference to RCD trip times why aim for a lower value unless you get TN values?

 

[Geordie Spark]

Hang on for a few minutes there lads ....................... I need to go for a pee and another bucket of popcorn.

 

[itselectric]

An earth rod resistance of 10 ohms, I think you must be living on the fens in Norfolk. I know some areas can produce a low reading,but you go to a place with made up land and you'd struggle to get anywhere near that.

 

[Engineer54]

I dont disagree up to a point and I think you will find Skelton doesnt either.
The issue constantly raised by us is that many who scoff at the Bs7671 max values appear to be happy with values which will still not operate an OCPD in a reasonable time, if at all.They are still reliant on an RCD.
Given that a value of 10 ohms or 200 ohms......or even 1667 ohms will make no difference to RCD trip times why aim for a lower value unless you get TN values?

Using your argument, One could easily ask WHY would you be giving a dam about Zs values on a TN system, if your both so confident that an RCD will ''Always'' fulfill your Zs requirement to trip the OCPD?? That argument (which i also disagree with) is just as valid as you blindly accepting a 200 ohm Ra on a TT system because a Reg vaguely say so ...

As i've stated many times here, if i had installed a rod position that gave me a 10 ohms value, another rod would ''always/automatically'' be added, you have would to be a bloody fool not too!! And yes, it is more than possible in many instances to achieve near TN values, but that takes a bit of time and effort and knowing what your doing. But again, that's something else that has been lost in time and space!! lol!!

Yes you can get over situations with the use of RCD devices, but at the end of the day, it is just a substitute, it's not really a valid TT system, even more so if you haven't even tried!!! I've been involved with far, far too many TT systems of every type and description, in various countries and locations, that you'll ever convince me in the slightest, that a rod with a 200 ohm Ra sitting on it, is any sort of a TT system. What it is, is a bloody joke!!

Encouraging these younger sparks and untrained so-called DI's, to not even bother installing a decent TT system and just bung an RCD in, isn't the message we should be portraying either!!

 

[Engineer54]

An earth rod resistance of 10 ohms, I think you must be living on the fens in Norfolk. I know some areas can produce a low reading,but you go to a place with made up land and you'd struggle to get anywhere near that.

Perhaps that's because you have been using these short thin twigs they now have the blatant cheek to call earth rods!! lol!! The Fens in Norfolk would easily give you TN values, if you know what your doing!! lol!!

 

[D Skelton]

Encouraging these younger sparks and untrained so-called DI's, to not even bother installing a decent TT system and just bung an RCD in, isn't the message we should be portraying either!!

No, quite right, but we should be encouraging them to read the regs and think for themselves instead of blindly following the advice given to them by anyone, myself included, on forums like this!

 

[Engineer54]

No, quite right, but we should be encouraging them to read the regs and think for themselves instead of blindly following the advice given to them by anyone, myself included, on forums like this!

Exactly what is the Regs going to tell them?? The section covering TT systems is in dire need of a complete overhaul. It after all hasn't given you (or anyone else) much meaningful guidance on this subject has it?? lol!!

 

[itselectric]

They used to recommend digging a pit and putting a large conductive plate in and connect your main earth to it.I'd like to see that done in some of the terraced houses on a TT system. that would Knock the price of a rewire up.

 

[D Skelton]

Exactly what is the Regs going to tell them?? The section covering TT systems is in dire need of a complete overhaul.

This is where we disagree. I think they're perfectly suitable and defines the needs of a TT system quite clearly. The only bit I'm not quite sure on is the 200 ohms? Seems like a randomly generated value and in my opinion should be replaced with a calculation.

It after all hasn't given you (or anyone else) much meaningful guidance on this subject has it?? lol!!

And this type of remark helps no one. If I didn't like you and respect your knowledge I would be calling you an arrogant twit right about now

 

[Jay Sparks]

I think this is one of the best, difference of opinion debates Ive seen for ages!!! Its what this forum has been lacking lately.
I dont have much to do with TT systems (since college) and there arent that many around here, or Ive just been lucky enough not come accross one but Im enjoying reading all the points raised by E54, wirepuller and D Skelton and its giving me a greater understanding of TT systems.

Jay

 

[Geordie Spark]

I think this is one of the best, difference of opinion debates Ive seen for ages!!! Its what this forum has been lacking lately.
I dont have much to do with TT systems (since college) and there arent that many around here, or Ive just been lucky enough not come accross one but Im enjoying reading all the points raised by E54, wirepuller and D Skelton and its giving me a greater understanding of TT systems.

Jay

I second that. It's all good stuff from these knowledgeable gentlemen.

 

[KAS1]

There's nothing wrong with NORFOLK........IMHO

 

[johnboy6083]

I'm with E54 on this. As he says, getting a low value in the first instance, will only lead to lower Ra values later on.
i understand Wirepullers points too, but I would prefer to have a system in which even if the RCD failed, there was a chance of another device operating, rather than not even worry about it.
I installed a rod (single rod, not needed, but was told to install anyway) on a job a year ago, it was 50 ohms.
i tested it the other week, whilst on the same site for another job and it was 2 ohms.
This is on top of a hill too.
I reckon another rod would see one ohm ish.

its nice to see a decent debate with decent technical discussion.

 

[itselectric]

I agree and I bet the've no bother finding a good earth path especiall along the broads

 

[itselectric]

Test it when we've had a long dry spell you could be looking at 50 ohms plus

 

[johnboy6083]

It was very dry when I installed the rod. Like E54 has mentioned, the ground around the rod consolidates, lowering the contact resistance.
the idea of installing multiple rods (on top of each other) is to get below the level where seasonal changes will have too much of an effect.

 

[Deleted member 9648]

]Using your argument, One could easily ask WHY would you be giving a dam about Zs values on a TN system, if your both so confident that an RCD will ''Always'' fulfill your Zs requirement to trip the OCPD?? That argument (which i also disagree with) is just as valid as you blindly accepting a 200 ohm Ra on a TT system because a Reg vaguely say so ...
[/B]
As i've stated many times here, if i had installed a rod position that gave me a 10 ohms value, another rod would ''always/automatically'' be added, you have would to be a bloody fool not too!! And yes, it is more than possible in many instances to achieve near TN values, but that takes a bit of time and effort and knowing what your doing. But again, that's something else that has been lost in time and space!! lol!!

Yes you can get over situations with the use of RCD devices, but at the end of the day, it is just a substitute, it's not really a valid TT system, even more so if you haven't even tried!!! I've been involved with far, far too many TT systems of every type and description, in various countries and locations, that you'll ever convince me in the slightest, that a rod with a 200 ohm Ra sitting on it, is any sort of a TT system. What it is, is a bloody joke!!

Encouraging these younger sparks and untrained so-called DI's, to not even bother installing a decent TT system and just bung an RCD in, isn't the message we should be portraying either!!

Firstly not all TN circuits will have RCD protection,and secondly there are maximum Ze values for TN systems,therefore if a Zs reading was considerably higher than the maximum for a standard OCPD something is clearly not right.That said I would have no problem relying on an RCD to meet disconnection times wher a type B needed to be changed for a type C/D and Zs slightly exceeded the max permitted value.

As I've already stated...if you achieve a TN value of Ra I'm with you all the way......but thinking 10...20...30 ohms is any better than 100-200 ohms ,I'm not.

 

[Deleted member 9648]

I'm with E54 on this. As he says, getting a low value in the first instance, will only lead to lower Ra values later on.
i understand Wirepullers points too, but I would prefer to have a system in which even if the RCD failed, there was a chance of another device operating, rather than not even worry about it.
I installed a rod (single rod, not needed, but was told to install anyway) on a job a year ago, it was 50 ohms.
i tested it the other week, whilst on the same site for another job and it was 2 ohms.
This is on top of a hill too.
I reckon another rod would see one ohm ish.

its nice to see a decent debate with decent technical discussion.

Just to make it clear....If you get a TN value it was worth the effort....my issue is ONLY with those who scoff at the Bs 7671 value of 200 ohms,and seem to think (for example) 20 ohms is the dogs danglies. It isnt. Do the calculations and you will find 20 ohms does the same job as 200....or 500 for that matter.

 

[Chr!s]

The most important factor is a stability, some soil/ground types won't allow a low Ra, that's just how it is. The tx arrangement plays a role too, you could have 21 ohms at the tx.

The key is a stable system for the soil type, if you have a favorable soil type and a combined HV/LV tx then yes a good low loop impedance is possible.

 

[itselectric]

It was very dry when I installed the rod. Like E54 has mentioned, the ground around the rod consolidates, lowering the contact resistance.
the idea of installing multiple rods (on top of each other) is to get below the level where seasonal changes will have too much of an effect.

I don't agree with the statement that the ground consolidates especially when (very rarely) we have a long dry spell and the ground starts to crack and gaps appear. I realise the rod is 1200 mm long and the ground won;t dry out for the full depth but it could go down about 300mm thus reducing the contact with the rod.